Pseudomorphic electronic and optoelectronic devices having planar contacts

ABSTRACT

In various embodiments, light-emitting devices incorporate smooth contact layers and polarization doping (i.e., underlying layers substantially free of dopant impurities) and exhibit high photon extraction efficiencies.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/046,515, filed Feb. 18, 2016, which is a continuation of U.S. patentapplication Ser. No. 14/208,379, filed Mar. 13, 2014, which claims thebenefit of and priority to U.S. Provisional Patent Application No.61/788,141, filed Mar. 15, 2013, the entire disclosure of each of whichis hereby incorporated herein by reference.

GOVERNMENT SUPPORT

This invention was made with United States Government support undercontract W911NF-09-2-0068 with the United States Army. The United StatesGovernment has certain rights in the invention.

TECHNICAL FIELD

In various embodiments, the present invention relates to improvingcarrier injection efficiency (e.g., the hole injection efficiency) intohigh-aluminum-content electronic and optoelectronic devices. Embodimentsof the present invention also relate to improving ultravioletoptoelectronic devices fabricated on nitride-based substrates, inparticular to improving light extraction therefrom.

BACKGROUND

The output powers, efficiencies, and lifetimes of short-wavelengthultraviolet light-emitting diodes (UV LEDs)—i.e., LEDs that emit lightat wavelengths less than 350 nm—based on the nitride semiconductorsystem remain limited due to high defect levels in the active region.These limitations are particularly problematic (and notable) in devicesdesigned to emit at wavelengths less than 280 nm. Particularly in thecase of devices formed on foreign substrates, such as sapphire, defectdensities remain high despite significant efforts to reduce them. Thesehigh defect densities limit both the efficiency and the reliability ofdevices grown on such substrates.

The recent introduction of low-defect, crystalline aluminum nitride(AlN) substrates has the potential to dramatically improve nitride-basedoptoelectronic semiconductor devices, particularly those having highaluminum concentration, due to the benefits of having lower defects inthe active regions of these devices. For example, UV LEDspseudomorphically grown on AlN substrates have been demonstrated to havehigher efficiencies, higher power, and longer lifetimes compared tosimilar devices formed on other substrates. Generally, thesepseudomorphic UV LEDs are mounted for packaging in a “flip-chip”configuration, where the light generated in the active region of thedevice is emitted through the AlN substrate, while the LED dies havetheir front surfaces (i.e., the top surfaces of the devices duringepitaxial growth and initial device fabrication prior to bonding) bondedto a patterned submount which is used to make electrical and thermalcontact to the LED chip. A good submount material is polycrystalline(ceramic) AlN because of the relatively good thermal expansion matchwith the AlN chip and because of the high thermal conductivity of thismaterial. Due to the high crystalline perfection that is achievable inthe active device region of such devices, internal efficiencies greaterthan 60% have been demonstrated.

Unfortunately, the photon-extraction efficiency is often still very poorin these devices, ranging from about 4% to about 15% achieved usingsurface-patterning techniques—much lower than exhibited by manyvisible-light (or “visible”) LEDs. Thus, the current generation ofshort-wavelength UV LEDs has low wall-plug efficiencies (WPEs) of, atbest, only a few percent, where the WPE is defined as the ratio ofusable optical power (in this case, emitted UV light) achieved from thediode to the electrical power supplied into the device. The WPE of anLED may be calculated by taking the product of the electrical efficiency(η_(cl)), the photon extraction efficiency (η_(ex)), and the internalefficiency (IE); i.e., WPE=η_(cl)×η_(hex)× IE. The IE itself is theproduct of current injection efficiency (η_(inj)) and the internalquantum efficiency (IQE); i.e., IE=η_(inj)×IQE. Thus, a low η_(hex) willdeleteriously impact the WPE even after the IE has been improved via thereduction of internal crystalline defects enabled by, e.g., the use ofthe AlN substrates referenced above as platforms for the devices.

There are several possible contributors to low photon-extractionefficiency. For example, currently available AlN substrates generallyhave some absorption in the UV wavelength range, even at wavelengthslonger than the band edge in AlN (which is approximately 210 nm). Thisabsorption tends to result in some of the UV light generated in theactive area of the device being absorbed in the substrate, hencediminishing the amount of light emitted from the substrate surface.However, this loss mechanism may be mitigated by thinning the AlN asdescribed in U.S. Pat. No. 8,080,833 (“the '833 patent,” the entiredisclosure of which is incorporated by reference herein) and/or byreducing the absorption in the AlN substrate as described in U.S. Pat.No. 8,012,257 (the entire disclosure of which is incorporated byreference herein). Additionally, UV LEDs typically suffer becauseapproximately 50% of the generated photons are directed toward thep-contact, which typically includes photon-absorbing p-GaN. Even whenphotons are directed toward the AlN surface, only about 9.4% typicallyescape from an untreated surface due to the large index of refraction ofthe AlN, which results in a small escape cone. These losses aremultiplicative and the average photon extraction efficiency may be quitelow.

As demonstrated in a recent publication by Grandusky et al. (James R.Grandusky et al., 2013 Appl. Phys. Express, Vol. 6, No. 3, 032101,hereinafter referred to as “Grandusky 2013,” the entire disclosure ofwhich is incorporated by reference herein), it is possible to increasethe photon extraction efficiency to approximately 15% in pseudomorphicUV LEDs grown on AlN substrates via the attachment of an inorganic (andtypically rigid) lens directly to the LED die via a thin layer of anencapsulant (e.g., an organic, UV-resistant encapsulant compound). Thisencapsulation approach, which is also detailed in U.S. patentapplication Ser. No. 13/553,093, filed on Jul. 19, 2012 (“the '093application,” the entire disclosure of which is incorporated byreference herein), increases the critical angle of total internalreflection through the top surface of the semiconductor die, whichsignificantly improves photon-extraction efficiency for the UV LEDs. Inaddition, and as mentioned above, the photon extraction efficiency maybe increased by thinning the AlN substrate and by roughening the surfaceof the AlN substrate surface as discussed in the '833 patent.

Unfortunately, none of these efforts addresses the major loss of photonsdue to absorption in the p-GaN utilized for the p-contact to thesedevices. In the type of pseudomorphic UV device described by Grandusky2013, p-GaN is used to make the p-contact to the LED because it allows arelatively low resistance contact to be made to the p-side of thedevice. However, the band gap energy of GaN is only 3.4 eV, and thus itis highly absorbing to photons with wavelengths shorter than 365 nm.Since typically 50% of the photons generated are directed toward thep-contact, these photons are typically immediately lost due toabsorption in the p-GaN. In addition, even photons directed toward theemission surface of the diode will typically only have a single chanceto escape since, if they are reflected back into the diode, they willlikely be absorbed by the p-GaN. The p-GaN is utilized conventionallybecause it is very difficult to make a low-resistivity contact top-Al_(x)Ga_(1-x)N where x is greater than 0.3. In addition, metals thatallow low-resistivity contact to the p-type nitride semiconductormaterial are generally poor reflectors. This reflectivity problem isparticularly exacerbated when the desired wavelength of the LED is lessthan 340 nm since most common metals will start to absorb strongly inthat regime.

In addition, prior work has suggested using a thick p-GaN layer (orp-Al_(x)Ga_(1-x)N layer with x<0.2) so that the hole current spreadssufficiently from and beneath the p-metal contacts. This approachgenerally will not work for devices emitting light of wavelengthsshorter than 300 nm because of the high absorption of the p-GaN orp-Al_(x)Ga_(1-x)N material at these shorter wavelengths.

Alternatively, the above-referenced shortcomings might be remedied viathe use of a non-absorbing p-type semiconductor on the p-side of the LEDand the use of p-contact metallurgy that reflects the UV photons.However, conventional approaches are unsuited to pseudomorphic UV LEDssince these approaches use multiple layers of thin p-Al_(x)Ga_(1-x)Nwhere the p-type Al_(x)Ga_(1-x)N layers are thin enough to be opticallytransparent to the UV radiation at wavelengths shorter than 300 nm. Thistype of multi-layer structure is very difficult to grow on apseudomorphic device structure (where the underlying substrate is eitherAlN or Al_(x)Ga_(1-x)N with x>0.6), because the large amount of strain(due to the lattice mismatch) typically causes the thin GaN (or lowaluminum content Al_(x)Ga_(1-x)N) to island and become very rough. Inthe Grandusky 2013 paper, contact roughening is addressed by making thep-type GaN layer quite thick; however, such layers, as detailed above,absorb UV photons and diminish UV LED device efficiencies.

Therefore, in view of the foregoing, there is a need for improvedcontact metallurgy and performance for UV LEDs, particularly those UVLEDs produced on AlN substrates, in order to improve characteristics,such as the WPE, of such devices.

SUMMARY

In various embodiments of the present invention, a smooth p-GaN (orp-Al_(x)Ga_(1-x)N layer where x<0.3) layer is produced on the activeregion (e.g., a pseudomorphic active region) of an electronic oroptoelectronic device grown on a single-crystal AlN substrate orsingle-crystal Al_(x)Ga_(1-x)N substrate where x>0.6. This smooth p-GaNor p-Al_(x)Ga_(1-x)N layer where x<0.3 will hereinafter be abbreviatedas the SPG layer. The SPG layer is very desirable for improvedfabrication of any pseudomorphic electronic or optoelectronic deviceutilizing a p-contact because it minimizes or substantially eliminatesthe rough surfaces that are difficult to etch and metallize uniformly.In various embodiments of the present invention, the SPG layer may alsobe made sufficiently thin to be transparent to UV radiation havingwavelengths shorter than 340 nm. The thin, UV-transparent SPG layer maybe combined with a reflective metal contact to the SPG layer, and thisbilayer structure may then be used to both efficiently inject holes intoa UV optoelectronic device and reflect UV photons from the p-contact. Invarious embodiments of the present invention, the thin, UV-transparentSPG layer, when combined with an appropriately designed UV reflectivecontact, will allow a pseudomorphic UV LED to be fabricated on an AlN(or Al_(x)Ga_(1-x)N substrate with x>0.6) substrate with a photonextraction efficiency greater than 25%. The thin SPG layer on apseudomorphic UV LED may be combined with a reflector metal contact toachieve a WPE greater than 10% at wavelengths shorter than 275 nm atcurrent densities exceeding 30 A/cm⁻².

In further embodiments of the present invention, a first metal layercapable of making a low-resistivity contact to the SPG layer is disposedon the SPG layer and patterned. The resulting gaps in the first metallayer may then be filled via the deposition of a second metal layer thatis an efficient reflector of UV photons. In this manner, the two-metalstructure provides the dual advantages of low contact resistance andhigh reflectivity, both of which improve the performance of UV LEDs.

In an exemplary embodiment, Al may be used as the reflector metal, as ithas >90% reflectivity to light having a wavelength of approximately 265nm. However, Al is quite poor for making a low-resistivity contact top-type GaN or p-type Al_(x)Ga_(1-x)N because of its low work function(4.26 eV). The high resistivity of the Al/nitride interface is addressedby the regions of the low-resistivity contact metal; however, in orderto prevent absorption of the UV photons by the contact metal, preferredembodiments of the invention utilize only limited contact areas betweenthe contact metal and the underlying semiconductor rather than a contactmetal-semiconductor contact area covering substantially all of thesemiconductor surface. For example, in some embodiments (i) more thanabout 10% of the semiconductor surface is covered by the contact metal,but (ii) less than about 70%, less than about 60%, less than about 50%,or even less than 40% of the semiconductor surface is covered by thecontact metal, while the remaining portion of the semiconductor surfaceis covered by the reflector metal to minimize deleterious absorption ofthe UV light.

In one aspect, embodiments of the invention feature a method of forminga contact to a UV light-emitting device. A substrate having anAl_(y)Ga_(1-y)N top surface is provided, where y≥0.4 (and ≤1.0). Thesubstrate may be substantially entirely composed of the Al_(y)Ga_(1-y)Nmaterial (e.g., AlN), or the substrate may include or consistessentially of a different material (e.g., silicon carbide, silicon,and/or sapphire) with the Al_(y)Ga_(1-y)N material formed thereover bye.g., epitaxial growth; such material may be substantially fully latticerelaxed and may have a thickness of, e.g., at least 1 μm. An active,light-emitting device structure is formed over the substrate, the devicestructure including or consisting essentially of a plurality of layerseach including or consisting essentially of Al_(x)Ga_(1-x)N. An undopedgraded Al_(1-z)Ga_(z)N layer is formed over the device structure, acomposition of the graded layer being graded in Ga concentration z suchthat the Ga concentration z increases in a direction away from thelight-emitting device structure. (For example, the Ga concentration zmay increase from a composition of approximately 0.15 proximate thedevice structure to a composition of approximately 1 at the top of thegraded layer.) A p-doped Al_(1-w)Ga_(w)N cap layer is formed over thegraded layer, the cap layer (i) having a thickness between approximately2 nm and approximately 30 nm, (ii) a surface roughness of less thanapproximately 6 nm over a sample size of approximately 200 μm×300 μm,and (iii) a Ga concentration w≥0.8. A metallic contact comprising atleast one metal is formed over the Al_(1-w)Ga_(w)N cap layer, themetallic contact having a contact resistivity to the Al_(1-w)Ga_(w)N caplayer of less than approximately 1.0 mΩ-cm².

Embodiments of the invention may include one or more of the following inany of a variety of combinations. Forming the Al_(1-w)Ga_(w)N cap layermay include or consist essentially of epitaxial growth at a temperaturebetween 850° C. and 900° C. and a growth pressure less than 50 Torr,e.g., between approximately 10 Torr and approximately 30 Torr, forexample 20 Torr. The Al_(1-w)Ga_(w)N cap layer may be doped with Mgand/or may be at least partially relaxed. The light-emitting device mayhave a photon extraction efficiency of greater than 25%. The gradedlayer and Al_(1-w)Ga_(w)N cap layer may collectively absorb less than80% of UV photons generated by the light-emitting device structure andhaving a wavelength less than 340 nm. The at least one metal of themetallic contact may include or consist essentially of Ni/Au and/or Pd.The metallic contact may have a reflectivity to light generated by thelight-emitting device structure of approximately 60% or less, or evenapproximately 30% or less. The metallic contact may be formed as aplurality of discrete lines and/or pixels of the at least one metal,portions of the Al_(1-w)Ga_(w)N cap layer not being covered by themetallic contact. A reflector may be formed over the metallic contactand the uncovered portions of the Al_(1-w)Ga_(w)N cap layer. Thereflector may include or consist essentially of a metal having greaterthan 60%, or even greater than 90%, reflectivity to UV light and a workfunction less than approximately 4.5 eV. The reflector may have acontact resistivity to the Al_(1-w)Ga_(w)N cap layer of greater thanapproximately 5 mΩ-cm², or even greater than approximately 10 mΩ-cm².The reflector may include or consist essentially of Al.

The light-emitting device may include or consist essentially of alight-emitting diode or a laser. A bottom portion of the graded layerproximate the active device structure may have a Ga concentration zsubstantially equal to a Ga concentration of a layer directlythereunder, and/or a top portion of the graded layer opposite the bottomportion of the graded layer may have a Ga concentration z ofapproximately 1. Forming the Al_(1-w)Ga_(w)N cap layer may include orconsist essentially of epitaxial growth at a growth rate between 0.5nm/min and 5 nm/min. Between forming the graded layer and forming theAl_(1-w)Ga_(w)N cap layer, a surface of the graded layer may be exposedto a precursor of the p-type dopant of the cap layer without exposure toa Ga precursor. The p-type dopant of the cap layer may include orconsist essentially of Mg. The substrate may consist essentially ofdoped or undoped AlN.

In another aspect, embodiments of the invention feature a UVlight-emitting device including or consisting essentially of a substratehaving an Al_(y)Ga_(1-y)N top surface, where y≥0.4 (and ≤1.0), alight-emitting device structure disposed over the substrate, the devicestructure including or consisting essentially of a plurality of layerseach including or consisting essentially of Al_(x)Ga_(1-x)N, an undopedgraded Al_(1-z)Ga_(z)N layer disposed over the device structure, acomposition of the graded layer being graded in Ga concentration z suchthat the Ga concentration z increases in a direction away from thelight-emitting device structure, a p-doped Al_(1-w)Ga_(w)N cap layerdisposed over the graded layer, the p-doped Al_(1-w)Ga_(w)N cap layer(i) having a thickness between approximately 2 nm and approximately 30nm, (ii) a surface roughness of less than approximately 6 nm over asample size of approximately 200 μm×300 μm, and (iii) a Ga concentrationw≥0.8, and a metallic contact disposed over the Al_(1-w)Ga_(w)N caplayer and including or consisting essentially of at least one metal, themetallic contact having a contact resistivity to the Al_(1-w)Ga_(w)N caplayer of less than approximately 1.0 mΩ-cm². The substrate may besubstantially entirely composed of the Al_(y)Ga_(1-y)N material (e.g.,AlN), or the substrate may include or consist essentially of a differentmaterial (e.g., silicon carbide, silicon, and/or sapphire) with theAl_(y)Ga_(1-y)N material formed thereover by e.g., epitaxial growth;such material may be substantially fully lattice relaxed and may have athickness of, e.g., at least 1 μm.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The Al_(1-w)Ga_(w)N cap layer may bedoped with Mg and/or may be at least partially relaxed. Thelight-emitting device may have a photon extraction efficiency of greaterthan 25%. The graded layer and Al_(1-w)Ga_(w)N cap layer maycollectively absorb less than 80% of UV photons generated by thelight-emitting device structure and having a wavelength less than 340nm. The at least one metal of the metallic contact may include orconsist essentially of Ni/Au and/or Pd. The metallic contact may have areflectivity to light generated by the light-emitting device structureof approximately 60% or less, or even approximately 30% or less.

The metallic contact may have the form of a plurality of discrete linesand/or pixels of the at least one metal, portions of the Al_(1-w)Ga_(w)Ncap layer not being covered by the metallic contact. A reflector may bedisposed over the metallic contact and the uncovered portions of theAl_(1-w)Ga_(w)N cap layer. The reflector may include or consistessentially of a metal having greater than 60%, or even greater than90%, reflectivity to UV light and a work function less thanapproximately 4.5 eV. The reflector may have a contact resistivity tothe Al_(1-w)Ga_(w)N cap layer of greater than approximately 5 mΩ-cm², oreven greater than approximately 10 mΩ-cm². The reflector may include orconsist essentially of Al. The light-emitting device may include orconsist essentially of a light-emitting diode or a laser. A bottomportion of the graded layer proximate the active device structure mayhave a Ga concentration z substantially equal to a Ga concentration of alayer directly thereunder, and/or a top portion of the graded layeropposite the bottom portion of the graded layer may have a Gaconcentration z of approximately 1. The substrate may consistessentially of doped or undoped AlN.

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations. As used herein, the term“substantially” means±10%, and in some embodiments, ±5%. The term“consists essentially of” means excluding other materials thatcontribute to function, unless otherwise defined herein. Nonetheless,such other materials may be present, collectively or individually, intrace amounts.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 is an optical profilometry surface-roughness scan of aconventional contact layer for an LED device;

FIG. 2 is an optical profilometry surface-roughness scan of a contactlayer for a light-emitting device in accordance with various embodimentsof the invention;

FIGS. 3A and 3B are schematic cross-sections of light-emitting devicesin accordance with various embodiments of the invention;

FIG. 4A is an atomic force microscopy scan of a capping layer for alight-emitting device in accordance with various embodiments of theinvention;

FIG. 4B is an atomic force microscopy scan of a conventional cappinglayer for a light-emitting device; and

FIG. 5 is a schematic cross-section of a portion of a light-emittingdevice in accordance with various embodiments of the invention.

DETAILED DESCRIPTION

Embodiments of the invention include pseudomorphic Al_(x)Ga_(1-x)Nelectronic and light-emitting devices on a substrate having anAl_(y)Ga_(1-y)N top surface, where y≥0.4 (and ≤1.0). The substrate maybe substantially entirely composed of the Al_(y)Ga_(1-y)N material(e.g., AlN), or the substrate may include or consist essentially of adifferent material (e.g., silicon carbide, silicon, and/or sapphire)with the Al_(y)Ga_(1-y)N material formed thereover by e.g., epitaxialgrowth; such material may be substantially fully lattice relaxed and mayhave a thickness of, e.g., at least 1 μm. (Although light-emittingdevices in accordance with preferred embodiments of the presentinvention are configured for the emission of UV light, the substrateneed not be transparent to UV radiation (e.g., silicon), since it may bepartially or substantially removed during device fabrication.) Thedevices according to embodiments of the invention also have a thin p-GaNor p-Al_(x)Ga_(1-x)N contact layer that is smooth (i.e., having aroot-mean-square (Rq) surface roughness of less than approximately 6 nm,or even less than approximately 1 nm). The roughness may characterizedwith optical profilometry over a sample size of approximately 200 μm×300μm, e.g., 233 μm×306.5 μm. FIG. 1 depicts a profilometry scan of aconventional rough contact-layer surface having an Rq value ofapproximately 33 nm. In contrast, FIG. 2 depicts a smooth contactsurface in accordance with embodiments of the present invention that hasan Rq value of only approximately 6 nm.

In preferred embodiments of the invention, the threading dislocationdensity (TDD) in the active region of the device is less than 10⁵ cm⁻².Furthermore, in preferred embodiments, the thin p-GaN orp-Al_(x)Ga_(1-x)N (SPG) final layer will be sufficiently thin to allowlight with wavelengths shorter than 340 nm to be transmitted withminimal absorption (i.e., absorption in a single pass no greater than80%, no greater than 50%, or even no greater than 40%). By decreasingthe thickness of the SPG layer or by increasing the concentration of Alin a given thickness for the SPG layer, the UV absorption at wavelengthsshorter than 340 nm may be decreased to 50%, to 25%, to 10%, or even to5% or less. For example, for a UV LED designed to operate at 265 nm, theabsorption coefficient of this radiation in the p-GaN layer will beapproximately 1.8×10⁵ cm⁻¹. Table 1 illustrates variousthickness-absorption relationships for Al_(x)Ga_(1-x)N layers of variousAl contents x and thicknesses for a variety of emission wavelengths. InTable 1, absorption values are shown for layers of 40% Al only foremission wavelengths up to 265 nm, as such layers become substantiallytransparent at larger wavelengths.

TABLE 1 % absorbed (single pass) Al % Emission in wavelength thickness(microns) AlGaN (nm) 0.001 0.002 0.003 0.01 0.025 0.05 0.1 0.2 0 2352.5% 4.9% 7.2% 22.1% 46.5% 71.3% 91.8% 99.3% 0 250 2.1% 4.1% 6.1% 18.9%40.8% 65.0% 87.8% 98.5% 0 265 1.7% 3.4% 5.1% 16.1% 35.4% 58.3% 82.6%97.0% 0 280 1.6% 3.1% 4.7% 14.8% 33.0% 55.1% 79.8% 95.9% 0 305 1.3% 2.6%3.8% 12.2% 27.7% 47.8% 72.7% 92.6% 20 235 1.8% 3.6% 5.4% 16.9% 37.0%60.3% 84.3% 97.5% 20 250 1.6% 3.1% 4.7% 14.8% 33.0% 55.1% 79.8% 95.9% 20265 1.3% 2.7% 4.0% 12.6% 28.6% 49.1% 74.1% 93.3% 20 280 1.2% 2.4% 3.5%11.3% 25.9% 45.1% 69.9% 90.9% 20 305 0.9% 1.8% 2.7%  8.6% 20.1% 36.2%59.3% 83.5% 40 235 1.4% 2.8% 4.1% 13.1% 29.5% 50.3% 75.3% 93.9% 40 2501.2% 2.4% 3.5% 11.3% 25.9% 45.1% 69.9% 90.9% 40 265 1.0% 2.0% 3.0%  9.5%22.1% 39.3% 63.2% 86.5%

In order to improve the photon extraction efficiency and enableextraction of photons directed towards the p-type material, a UVreflector may be introduced into the device structure to reflecttransmitted photons and direct them towards the AlN substrate so thatthey may be extracted from the device. In visible LEDs, this is oftenaccomplished by using a silver p-contact, as silver both forms an ohmiccontact to visible-LED structures and is reflective to visible photons.In addition, the layers that form a visible LED are generallytransparent to the photons being generated in the quantum wells.However, the reflectivity of silver drops rapidly in the UV range. Thereflectivities of most other common metals also drop as the wavelengthdecreases into the UV range with the exception of Al, whichunfortunately does not form a good ohmic contact to p-type GaN orAl_(x)Ga_(1-x)N.

Thus, in order to reflect photons while still achieving good ohmiccontact, a fairly non-reflective (at least to UV photons) contactmetallurgy (e.g., Ni/Au or Pd) may be formed over the contact layer butpatterned to reduce the surface “footprint” of the contact over thesemiconductor. In this manner, the surface area over the device layersthat is non-reflective to UV photons is minimized, yet good ohmiccontact to the semiconductor is still achieved. In order to reflect atleast a portion of the UV photons, a reflective metal such as Al may beprovided directly over the semiconductor between the non-reflectivecontact regions. The reflective metal makes an ohmic contact with thenon-reflective metal, enabling electrical contact to the LED whileutilizing the superior metal-semiconductor contact formed by thenon-reflective metal.

In such embodiments, the SPG layer may include or consist essentially ofa p-GaN or p-Al_(x)Ga_(1-x)N layer where x<0.3. Typically, thicker SPGlayers may be utilized as the Ga content is decreased, as thelattice-mismatch strain (that may roughen the SPG layer) between the SPGlayer and the underlying AlN substrate decreases. However, the Gacontent of the SPG layer is preferably maintained at or above 70% inorder to enable a highly doped, low-resistivity layer.

For p-type Al_(x)Ga_(1-x)N layers doped with Mg, as the Al mole fraction(x) is increased, the activation energy of the Mg impurity is increased.This leads to lower activation of the Mg, resulting in lower holeconcentration as the Al mole fraction is increased. One solution to thisis to utilize polarization-induced doping, which may be achieved by thegrading of an Al_(x)Ga_(1-x)N layer from high x to lower x as it isdeposited. This may be used to achieve hole concentrations much higherthan may be achieved through conventional impurity doping. In addition,this technique may result in improved carrier mobilities due to lack ofimpurity scattering and reduced temperature dependence of the holeconcentration. High hole concentrations may be achieved in the absenceof impurity doping or in addition to impurity doping. Preferredembodiments of the invention feature low dislocation density inpseudomorphic graded layers, which enables high hole concentration inthe absence of impurity doping, thus allowing for higher conductivityand improved current spreading from thin transparent layers. These highhole concentrations make it possible to achieve p-contacts with lowresistivity. In particular, resistivities less than 10 mΩ-cm² may beachieved in accordance with embodiments of the present invention. Inpreferred embodiments, resistivities less than 5 mΩ-cm² are achieved andutilized in UV LEDs. For contacts with resistivities of 10 mΩ-cm², thedevice may be operated at 30 A/cm² with a 1:3 ratio of contact metal toreflector metal (as detailed above) and achieve a voltage drop acrossthe p-contact of less than 1.2 V with a device area of 0.0033 cm². Bycovering 75% of the p-contact area with good reflector metal and usingan SPG layer with absorption less than 80%, it is possible to achievephoton extraction efficiencies in UV LEDs that are greater than 25%,particularly when combined with the efficient photon extractiontechniques described above. When the high photon extraction efficiencyof greater than 25% is combined with the low-resistivity contactdescribed above, embodiments of the invention exhibit wall plugefficiencies greater than 10% at an operating current density exceeding30 A/cm².

FIG. 3A depicts a pseudomorphic UV light emitting diode (“PUVLED”)structure 300 in accordance with embodiments of the present invention. Asemiconductor substrate 305, which includes or consists essentially of,e.g., a substrate having an Al_(y)Ga_(1-y)N top surface, where y≥0.4(and ≤1.0), is provided. The substrate may be substantially entirelycomposed of the Al_(y)Ga_(1-y)N material (e.g., AlN), or the substratemay include or consist essentially of a different material (e.g.,silicon carbide, silicon, and/or sapphire) with the Al_(y)Ga_(1-y)Nmaterial formed thereover by e.g., epitaxial growth; such material maybe substantially fully lattice relaxed and may have a thickness of,e.g., at least 1 μm. As mentioned above, the substrate 305 need not betransparent to UV radiation (e.g., silicon), since it may be partiallyor substantially removed during device fabrication. Semiconductorsubstrate 305 may be miscut such that the angle between its c-axis andits surface normal is between approximately 0° and approximately 4°. Ina preferred embodiment, the misorientation of the surface ofsemiconductor substrate 305 is less than approximately 0.3°, e.g., forsemiconductor substrates 305 that are not deliberately or controllablymiscut. In other embodiments, the misorientation of the surface ofsemiconductor substrate 305 is greater than approximately 0.3°, e.g.,for semiconductor substrates 305 that are deliberately and controllablymiscut. In a preferred embodiment, the direction of the miscut istowards the a-axis. The surface of semiconductor substrate 305 may havea group-III (e.g., Al—) polarity or N-polarity, and may be planarized,e.g., by chemical-mechanical polishing. The RMS surface roughness ofsemiconductor substrate is preferably less than approximately 0.5 nm fora 10 μm×10 μm area. In some embodiments, atomic-level steps aredetectable on the surface when probed with an atomic-force microscope.The threading dislocation density of semiconductor substrate 305 may bemeasured using, e.g., etch pit density measurements after a 5 minuteKOH—NaOH eutectic etch at 450° C. Preferably the threading dislocationdensity is less than approximately 2×10³ cm⁻². In some embodimentssubstrate 305 has an even lower threading dislocation density.Semiconductor substrate 305 may be topped with a homoepitaxial layer(not shown) that includes or consists essentially of the samesemiconductor material present in semiconductor substrate 300, e.g.,AlN.

In an embodiment, an optional graded buffer layer 310 is formed onsemiconductor substrate 305. Graded buffer layer 310 may include orconsist essentially of one or more semiconductor materials, e.g.,Al_(x)Ga_(1-x)N. In a preferred embodiment, graded buffer layer 310 hasa composition approximately equal to that of semiconductor substrate 305at an interface therewith in order to promote two-dimensional growth andavoid deleterious islanding (such islanding may result in undesiredelastic strain relief and/or surface roughening in graded buffer layer310 and subsequently grown layers). The composition of graded bufferlayer 310 at an interface with subsequently grown layers (describedbelow) is generally chosen to be close to (e.g., approximately equal to)that of the desired active region of the device (e.g., theAl_(x)Ga_(1-x)N concentration that will result in the desired wavelengthemission from the PUVLED). In an embodiment, graded buffer layer 310includes Al_(x)Ga_(1-x)N graded from an Al concentration x ofapproximately 100% to an Al concentration x of approximately 60%.

A bottom contact layer 320 is subsequently formed above substrate 305and optional graded layer 310, and may include or consist essentially ofAl_(x)Ga_(1-x)N doped with at least one impurity, e.g., Si. In anembodiment, the Al concentration x in bottom contact layer 320 isapproximately equal to the final Al concentration x in graded layer 310(i.e., approximately equal to that of the desired active region(described below) of the device). Bottom contact layer 320 may have athickness sufficient to prevent current crowding after devicefabrication (as described below) and/or to stop on during etching tofabricate contacts. For example, the thickness of bottom contact layer320 may be less than approximately 200 nm. When utilizing a bottomcontact layer 320 of such thickness, the final PUVLED may be fabricatedwith back-side contacts. In many embodiments, bottom contact layer 320will have high electrical conductivity even with a small thickness dueto the low defect density maintained when the layer is pseudomorphic. Asutilized herein, a pseudomorphic film is one where the strain parallelto the interface is approximately that needed to distort the lattice inthe film to match that of the substrate. Thus, the parallel strain in apseudomorphic film will be nearly or approximately equal to thedifference in lattice parameters between an unstrained substrateparallel to the interface and an unstrained epitaxial layer parallel tothe interface.

A multiple-quantum well (“MQW”) layer 330 is fabricated above bottomcontact layer 320. MQW layer 330 corresponds to the “active region” ofPUVLED structure 300 and includes a plurality of quantum wells, each ofwhich may include or consist essentially of AlGaN. In an embodiment,each period of MQW layer 330 includes an Al_(x)Ga_(1-x)N quantum welland an Al_(y)Ga_(1-y)N barrier, where x is different from y. In apreferred embodiment, the difference between x and y is large enough toobtain good confinement of the electrons and holes in the active region,thus enabling high ratio of radiative recombination to non-radiativerecombination. In an embodiment, the difference between x and y isapproximately 0.05, e.g., x is approximately 0.35 and y is approximately0.4. However, if the difference between x and y is too large, e.g.,greater than approximately 0.3, deleterious islanding may occur duringformation of MQW layer 330. MQW layer 330 may include a plurality ofsuch periods, and may have a total thickness less than approximately 50nm. Above MQW layer 330 may be formed an optional thin electron-blocking(or hole-blocking if the n-type contact is put on top of the device)layer 340, which includes or consists essentially of, e.g.,Al_(x)Ga_(1-x)N, which may be doped with one or more impurities such asMg. Electron-blocking layer 340 has a thickness that may range between,e.g., approximately 10 nm and approximately 50 nm. A top contact layer350 is formed above electron blocking layer 340, and includes orconsists essentially of one or more semiconductor materials, e.g.,Al_(x)Ga_(1-x)N, doped with at least one impurity such as Mg. Topcontact layer 350 is doped either n-type or p-type, but withconductivity opposite that of bottom contact layer 310. The thickness oftop contact layer 350 is, e.g., between approximately 50 nm andapproximately 100 nm. Top contact layer 350 is capped with a cap layer360, which includes or consists essentially of one or more semiconductormaterials doped with the same conductivity as top contact layer 350. Inan embodiment, cap layer 360 includes GaN doped with Mg, and has athickness between approximately 10 nm and approximately 200 nm,preferably approximately 50 nm. In some embodiments, high-quality ohmiccontacts may be made directly to top contact layer 350 and cap layer 360is omitted. In other embodiments, top contact layer 350 and/orelectron-blocking layer 340 are omitted and the top contact is formeddirectly on cap layer 360 (in such embodiments, cap layer 360 may beconsidered to be a “top contact layer”). While it is preferred thatlayers 310-340 are all pseudomorphic, top contact layer 350 and/or caplayer 360 may relax without introducing deleterious defects into theactive layers below which would adversely affect the performance ofPUVLED structure 300 (as described below with reference to FIG. 3B).Each of layers 310-350 is pseudomorphic, and each layer individually mayhave a thickness greater than its predicted critical thickness.Moreover, the collective layer structure including layers 310-350 mayhave a total thickness greater than the predicted critical thickness forthe layers considered collectively (i.e., for a multiple-layerstructure, the entire structure has a predicted critical thickness evenwhen each individual layer would be less than a predicted criticalthickness thereof considered in isolation).

In various embodiments, layers 310-340 of PUVLED structure 300 arepseudomorphic, and cap layer 360 is intentionally relaxed. As shown inFIG. 3B, layers 310-340 are formed as described above with reference toFIG. 3A. Cap layer 360 is subsequently formed in a partially orsubstantially strain-relaxed state via judicious selection of itscomposition and/or the deposition conditions. For example, the latticemismatch between cap layer 360 and substrate 305 and/or MQW layer 330may be greater than approximately 1%, greater than approximately 2%, oreven greater than approximately 3%. In a preferred embodiment, cap layer360 includes or consists essentially of undoped or doped GaN, substrate305 includes or consists essentially of AlN, and MQW layer 330 includesor consists essentially of multiple Al_(0.55)Ga_(0.45)N quantum wellsinterleaved with Al_(0.75)Ga_(0.25)N barrier layers, and cap layer 360is lattice mismatched by approximately 2.4%. Cap layer 360 may besubstantially relaxed, i.e., may have a lattice parameter approximatelyequal to its theoretical unstrained lattice constant. As shown, apartially or substantially relaxed cap layer 360 may containstrain-relieving dislocations 370 having segments threading to thesurface of cap layer 360 (such dislocations may be termed “threadingdislocations”). The threading dislocation density of a relaxed cap layer360 may be larger than that of substrate 305 and/or layers 310-340 by,e.g., one, two, or three orders of magnitude, or even larger. Cap layer360 is preferably not formed as a series of coalesced or uncoalescedislands, as such islanding may deleteriously impact the surfaceroughness of cap layer 360.

A graded layer may be formed between layers 310-340 and cap layer 360,and its composition at its interfaces with layers 340, 360 maysubstantially match the compositions of those layers. The thickness ofthis graded layer, which is preferably pseudomorphically strained, mayrange between approximately 10 nm and approximately 50 nm, e.g.,approximately 30 nm. In some embodiments, epitaxial growth may betemporarily stopped between growth of the graded layer and cap layer360.

In an exemplary embodiment, an electron-blocking layer 340 including orconsisting essentially of Al_(0.8)Ga_(0.2)N or Al_(0.85)Ga_(0.15)N isformed over MQW layer 330. Prior to formation of cap layer 360 includingor consisting essentially of GaN, a graded layer is formed overelectron-blocking layer 340. The graded layer may be graded incomposition from, for example, Al_(0.85)Ga_(0.15)N to GaN over athickness of approximately 30 nm. The graded layer may be formed by,e.g., MOCVD, and in this embodiment is formed by ramping the flow of TMAand TMG (by ramping the flow of hydrogen through their respectivebubblers) from the conditions utilized to form electron-blocking layer340 to 0 standard cubic centimeters per minute (sccm) and 6.4 sccm,respectively, over a period of approximately 24 minutes, thus resultingin a monotonic grade from Al_(0.85)Ga_(0.15)N to GaN (all of the othergrowth conditions are substantially fixed). The thickness of the gradedlayer in this exemplary embodiment is approximately 30 nm, and a holeconcentration of approximately 3×10¹⁹ cm⁻³ may be achieved throughpolarization doping without impurity doping (e.g., even beingsubstantially free of doping impurities), as modeled using SiLENSesoftware. In general, polarization doping is enabled by the polarizationin nitride materials that is due to the difference in electronegativitybetween the metal atoms and the nitrogen atoms. This results in apolarization field along asymmetric directions in the wurtzite crystalstructure. In addition, strain in the layers may result in additionalpiezoelectric polarization fields and thus additional polarizationdoping. These fields create fixed charges at abrupt interfaces (e.g.,two-dimensional sheets) or graded composition layers (e.g.,three-dimensional volumes), which results in mobile carriers of theopposite sign. The magnitude of the total charge is defined by thedifference in Al compositions within the graded layer, i.e., thedifference between the starting composition and the final composition.The concentration of carriers is defined by the total charge divided bythe graded layer thickness. A very high carrier concentration may beachieved by a high composition change over a small thickness, while alower composition change or larger grading thickness typically resultsin a smaller carrier concentration; however, for a given compositionchange the total number of carriers is generally constant.

As detailed above, preferred embodiments of the present inventionutilize very thin SPG layers in order to minimize absorption of UVphotons therein. Such SPG layers preferably have thicknesses of lessthan 50 nm, e.g., between approximately 10 nm and approximately 30 nm.In an embodiment, smooth (25-50 nm) p-GaN layers were grown on a typicalpseudomorphic LED structure (AlN/n-AlGaN/MQW/electron-blockinglayer/p-GaN) by MOCVD and trimethylgallium (TMGa) and NH₃ were used asGa and N precursors. Some conventional p-GaN layers are grown at 1000°C. and at a pressure of 100 Torr, and often these layers are rough,exhibiting an islanded or pyramidal morphology. Such approaches areencouraged by the conventional wisdom in the art, which indicates thatone should enhance the mobility of the Ga adatom to promote lateralgrowth and coalescence of the layer. Thus, conventional wisdom teachesthat contact-layer growth should use increased V/III ratios and highertemperatures. However, such techniques were unable to achieve smoothsurfaces on the pseudomorphic layer in the thickness range utilized inembodiments of the present invention. Notably, the large strain in thepseudomorphic layer enhances island formation and increased surfaceroughness. Unexpectedly, in order to suppress such surface roughening,in accordance with embodiments of the present invention, growthtemperatures of 850° C.-900° C. may be utilized for growth of the SPGlayer, and growth pressures of 20 Torr may be utilized to enhance theadatom mobility at this lower growth-temperature regime. The growth rateof smooth p-GaN is only approximately 5 nm/min. The morphological andelemental properties of resulting SPG layers were investigated usingatomic force microscopy (AFM) and secondary ion mass spectroscopy(SIMS). AFM shows smoother p-GaN layers (Rq value of approximately 0.85nm) as shown in FIG. 4A, compared to the rougher morphology ofconventional p-GaN (Rq value of approximately 7.2 nm) shown in FIG. 4B.Here, the actual island heights are over 50 nm and these thicker islandsresult in higher absorption and also leave areas uncovered by p-GaNwhich will result in poor p-contact by the contact metallization whenthese holes occur in the regions that are covered by the contactmetallization. SIMS analysis shows higher doping concentration (by afactor of two) in the smooth p-GaN compared to the conventional p-GaN;however, the concentration is not constant and does not reachequilibrium until growth of ˜25 nm of p-GaN, resulting in difficultiesmaking ohmic contacts to layers thinner than 25 nm. In order to overcomethis issue, a soak, i.e., exposure within the deposition chamber, (of,e.g., 1-10 minutes, for example 5 minutes) with only the dopant (e.g.,Mg) source (i.e., no Ga source) flowing may be utilized to saturate thesurface prior to growth initiation. For example,bis-cyclopentadienylmagnesium (Cp2Mg) may be utilized at an Mg sourcefor the soak when MOCVD is being utilized for layer growth. Theprecursor may be disposed within a bubbler, and a carrier gas such asnitrogen or hydrogen may be flowed into the bubbler to form a gassolution saturated with the dopant precursor. This enables higher dopantconcentration and good ohmic contact formation to layers as thin as 5nm. In summary, very thin p-GaN layers (<10 nm) may be easily realizedin this growth regime owing to the slower growth rate and the conformalmorphology while the doping concentrations may be optimized by adjustingthe input precursor flows.

In an exemplary embodiment, polarization doping and a thin SPG layer arecombined with a patterned reflector as shown in FIG. 5, which depicts aportion of a UV LED device 500. In device 500, region 510 includes orconsists essentially of the AN substrate and the active region of thedevice, for example as detailed above and illustrated in FIG. 3A. Region510 is topped with a SPG layer 520, which is kept smooth to enable avery thin layer with high UV transparency. A contact layer 530, formedon the SPG layer 520, is typically substantially not UV reflective butforms a good ohmic contact to the SPG layer 520. In an exemplaryembodiment, contact layer 530 includes or consists essentially of Ni/Au.As shown, the contact layer is, in preferred embodiments, patterned ontothe surface of SPG layer 520. The spacing between individual portions ofcontact layer 530 may be defined via, e.g., conventionalphotolithography. The pattern may be in the form of lines or patterns ofisolated “pixels” (or “islands”) as shown in FIG. 5. Lines may havewidths of, for example, 1 μm to 50 μm, e.g., 5 μm, and may have spacingstherebetween of, for example 1 μm to 50 μm, e.g., 5 μm. Pixels may be,for example, substantially cubic or rectangular solids or may even besubstantially hemispherical, and pixels may have a dimension such aswidth, length, or diameter of, for example, 1 μm to 50 μm, e.g., 5 μm.The contact area and the spacing are typically defined to optimize thewall plug efficiency of the device.

As shown in FIG. 5, the contact layer 530 may be capped with a reflector540 formed both above the contact layer 530 (or isolated portionsthereof) and between portions of the contact layer 530 (i.e., in directcontact with SPG layer 520). The reflector 540 typically includes orconsists essentially of a metal (or metal alloy) that is highlyreflective to UV light but that does not form a good ohmic contact tothe SPG layer 520. For example, the reflector 540 may include or consistessentially of Al. The contact area of the contact layer 530 willgenerally determine, at least in part, the effective contact resistanceof the combined contact layer 530 and reflector 540. For instance, if10% of the area is covered by the contact layer 530, then the effectivecontact resistance is increased by a factor of ten. However, at the sametime, the reflector area (i.e., the area of SPG layer 520 cappeddirectly by reflector 540, without contact layer 530 therebetween) isincreased. In an exemplary embodiment, the contact resistivity of thecontact layer 530 is less than approximately 1.0 mΩ-cm², or even lessthan approximately 0.5 mΩ-cm². By using a 1:10 ratio of contact 530 areato reflector 540 area, the effective contact resistance is increased to5 mΩ and the effective (averaged over all area) reflector is reduced by10% (e.g., a 90% reflectivity of the reflector 540 is effectivelyreduced to 81%). In addition, the size of individual metal contactpixels of contact layer 530 is preferably kept as small as possible sothat current spreading from the individual contact pixels occurs. Thisincreases the probability that the generated photons will strike thereflector 540 rather than the contact pixel of contact layer 530 (whichwould typically occur if the current traveled straight down from thecontact metal pixel of contact layer 530). The polarization-doped AlGaNenables current spreading while maintaining transparency even with athin SPG layer 520; the thin SPG layer 520 is used primarily to lowerthe contact resistance while maintaining low absorption. This is indirect contrast to conventional methods where p-doping in high Alcontent Al_(x)Ga_(1-x)N is highly resistive and will not allow currentspreading.

Embodiments of the invention may utilize photon-extraction techniquesdescribed in the '093 application. Such techniques include surfacetreatment (e.g., roughening, texturing, and/or patterning), substratethinning, substrate removal, and/or the use of rigid lenses with thinintermediate encapsulant layers. Exemplary substrate-removal techniquesinclude laser lift-off, as described in “High brightness LEDs forgeneral lighting applications using the new Thin GaN™-Technology”, V.Haerle, et al., Phys. Stat. Sol. (a) 201, 2736 (2004), the entiredisclosure of which is incorporated by reference herein.

In embodiments in which the device substrate is thinned or removed, theback surface of the substrate may be ground, for example, with a 600 to1800 grit wheel. The removal rate of this step may be purposefullymaintained at a low level (approximately 0.3-0.4 μm/s) in order to avoiddamaging the substrate or the device layers thereover. After theoptional grinding step, the back surface may be polished with apolishing slurry, e.g., a solution of equal parts of distilled water anda commercial colloidal suspension of silica in a buffered solution ofKOH and water. The removal rate of this step may vary betweenapproximately 10 μm/min and approximately 15 μm/min. The substrate maybe thinned down to a thickness of approximately 200 μm to approximately250 μm, or even to a thickness of approximately 20 μm to approximately50 μm, although the scope of the invention is not limited by this range.In other embodiments, the substrate is thinned to approximately 20 μm orless, or even substantially completely removed. The thinning step ispreferably followed by wafer cleaning in, e.g., one or more organicsolvents. In one embodiment of the invention, the cleaning step includesimmersion of the substrate in boiling acetone for approximately 10minutes, followed by immersion in boiling methanol for approximately 10minutes.

Structures fabricated utilizing the above-described techniques inaccordance with various embodiments of the present invention have beenfabricated with three different reflector metal areas, 0%, 51%, and 60%.No significant forward voltage increase was observed at 51% reflectormetal area with only 0.1 V increase at 100 mA (while a 0.4 V increasewas seen at 60% reflector metal area), and an improvement in extractionefficiency of 24% was measured for devices emitting through a thickabsorbing AlN substrate with 51% reflector metal area. However, whencombined with die thinning, roughening, and encapsulation an overallgain of ˜100% was achieved for devices with 51% reflector metal areacompared to devices with 0% reflector area. The results from 60%reflector area were improved less than 51%, but optimization of bothcontact metal spacing and reflector area may result in further gains inoverall efficiency.

The terms and expressions employed herein are used as terms ofdescription and not of limitation, and there is no intention, in the useof such terms and expressions, of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed.

What is claimed is:
 1. A method of forming an ultraviolet (UV)light-emitting device, the method comprising: providing a Al_(y)Ga_(1-y)N substrate, wherein, at least at a top surface of the substrate,1.0≥y≥0.4; forming a light-emitting device structure over the substrate,the device structure comprising a plurality of layers each comprisingAl_(x)Ga_(1-x)N; forming an undoped graded Al_(1-z)Ga_(z)N layer overthe device structure, a composition of the graded layer being graded inGa concentration z such that the Ga concentration z increases in adirection away from the light-emitting device structure; forming overthe graded layer a p-doped Al_(1-w)Ga_(w)N cap layer having a Gaconcentration w, wherein 1.0≥w≥0.7; and forming over the Al_(1-w)Ga_(w)Ncap layer a metallic contact comprising at least one metal, wherein themetallic contact is formed as a plurality of discrete lines and/orpixels of the at least one metal, portions of the Al_(1-w)Ga_(w)N caplayer not being covered by the metallic contact.
 2. The method of claim1, wherein forming the Al_(1-w)Ga_(w)N cap layer comprises epitaxialgrowth at a temperature between 850° C. and 900° C. and a growthpressure less than 50 Torr.
 3. The method of claim 2, wherein the growthpressure is approximately 20 Torr.
 4. The method of claim 1, wherein thelight-emitting device has a photon extraction efficiency of greater than25%.
 5. The method of claim 1, wherein the at least one metal of themetallic contact comprises at least one of Ni/Au or Pd.
 6. The method ofclaim 1, wherein the metallic contact has a reflectivity to lightgenerated by the light-emitting device structure of approximately 60% orless.
 7. The method of claim 1, wherein the metallic contact has areflectivity to light generated by the light-emitting device structureof approximately 30% or less.
 8. The method of claim 1, furthercomprising forming a reflector over the metallic contact and theuncovered portions of the Al_(1-w)Ga_(w)N cap layer.
 9. The method ofclaim 8, wherein the reflector comprises a metal having greater than 90%reflectivity to UV light and a work function less than approximately 4.5eV.
 10. The method of claim 8, wherein the reflector has a contactresistivity to the Al_(1-w)Ga_(w)N cap layer of greater thanapproximately 5 mΩ-cm².
 11. The method of claim 8, wherein the reflectorhas a contact resistivity to the Al_(1-w)Ga_(w)N cap layer of greaterthan approximately 10 mΩ-cm².
 12. The method of claim 8, wherein thereflector comprises Al.
 13. The method of claim 1, wherein thelight-emitting device comprises a light-emitting diode.
 14. The methodof claim 1, wherein a bottom portion of the graded layer proximate thedevice structure has a Ga concentration z substantially equal to a Gaconcentration of a layer directly thereunder.
 15. The method of claim 1,wherein a top portion of the graded layer proximate the Al_(1-w)Ga_(w)Ncap layer has a Ga concentration z of approximately
 1. 16. The method ofclaim 1, wherein the substrate comprises doped or undoped AIN.
 17. Themethod of claim 1, wherein the Al_(1-w)Ga_(w)N cap layer has a thicknessbetween approximately 2 nm and approximately 30 nm.
 18. The method ofclaim 1, wherein the Al_(1-w)Ga_(w)N cap layer has a surface roughnessof less than approximately 6 nm over a sample size of approximately 200μm×300 μm.
 19. The method of claim 1, wherein 1.0≥w≥0.8.
 20. The methodof claim 1, wherein the metallic contact has a contact resistivity tothe Al_(1-w)Ga_(w)N cap layer of less than approximately 1.0 mΩ-cm². 21.The method of claim 1, wherein a thickness of the undoped graded layerranges from 10 nm to 50 nm.
 22. The method of claim 1, wherein thethickness of the undoped graded layer ranges from 30 nm to 50 nm.
 23. Amethod of forming an ultraviolet (UV) light-emitting device, the methodcomprising: providing a Al_(y)Ga_(1-y)N substrate, wherein, at least ata top surface of the substrate, 1.0≥y≥0.4; forming a light-emittingdevice structure over the substrate, the device structure comprising aplurality of layers each comprising Al_(x)Ga_(1-x)N; forming an undopedgraded Al_(1-z)Ga_(z)N layer over the device structure, a composition ofthe graded layer being graded in Ga concentration z such that the Gaconcentration z increases in a direction away from the light-emittingdevice structure; forming over the graded layer a p-dopedAl_(1-w)Ga_(w)N cap layer having a Ga concentration w, wherein1.0≥w≥0.7; and forming over the Al_(1-w)Ga_(w)N cap layer a metalliccontact comprising at least one metal, wherein the graded layer andAl_(1-w)Ga_(w)N cap layer collectively absorb less than 80% of UVphotons generated by the light-emitting device structure and having awavelength less than 340 nm.
 24. The method of claim 23, wherein themetallic contact has a reflectivity to light generated by thelight-emitting device structure of approximately 60% or less.
 25. Themethod of claim 23, wherein the metallic contact has a reflectivity tolight generated by the light-emitting device structure of approximately30% or less.
 26. The method of claim 23, wherein a thickness of theundoped graded layer ranges from 10 nm to 50 nm.
 27. The method of claim23, wherein the thickness of the undoped graded layer ranges from 30 nmto 50 nm.
 28. A method of forming an ultraviolet (UV) light-emittingdevice, the method comprising: providing a Al_(y)Ga_(1-y)N substrate,wherein, at least at a top surface of the substrate, 1.0≥y≥0.4; forminga light-emitting device structure over the substrate, the devicestructure comprising a plurality of layers each comprisingAl_(x)Ga_(1-x)N; forming an undoped graded Al_(1-z)Ga_(z)N layer overthe device structure, a composition of the graded layer being graded inGa concentration z such that the Ga concentration z increases in adirection away from the light-emitting device structure; forming overthe graded layer a p-doped Al_(1-w)Ga_(w)N cap layer having a Gaconcentration w, wherein 1.0≥w≥0.7; and forming over the Al_(1-w)Ga_(w)Ncap layer a metallic contact comprising at least one metal, wherein theundoped graded layer is pseudomorphically strained.
 29. The method ofclaim 28, wherein a thickness of the undoped graded layer ranges from 10nm to 50 nm.
 30. The method of claim 28, wherein the light-emittingdevice has a photon extraction efficiency of greater than 25%.
 31. Themethod of claim 28, wherein the graded layer and Al_(1-w)Ga_(w)N caplayer collectively absorb less than 80% of UV photons generated by thelight-emitting device structure and having a wavelength less than 340nm.
 32. The method of claim 28, wherein the at least one metal of themetallic contact comprises at least one of Ni/Au or Pd.
 33. The methodof claim 28, wherein the metallic contact has a reflectivity to lightgenerated by the light-emitting device structure of approximately 30% orless.
 34. The method of claim 28, wherein the metallic contact is formedas a plurality of discrete lines and/or pixels of the at least onemetal, portions of the Al_(1-w)Ga_(w)N cap layer not being covered bythe metallic contact.
 35. The method of claim 34, further comprisingforming a reflector over the metallic contact and the uncovered portionsof the Al_(1-w)Ga_(w)N cap layer.
 36. The method of claim 35, whereinthe reflector comprises a metal having greater than 90% reflectivity toUV light and a work function less than approximately 4.5 eV.
 37. Themethod of claim 35, wherein the reflector has a contact resistivity tothe Al_(1-w)Ga_(w)N cap layer of greater than approximately 5 mΩ-cm².38. The method of claim 35, wherein the reflector comprises Al.
 39. Themethod of claim 28, wherein the thickness of the undoped graded layerranges from 30 nm to 50 nm.